Abstract
Chloride conductance disturbances contribute to sarcolemmal dysfunction in myotonic dystrophy type 1 (DM1) and type 2 (DM2). Studies using muscle velocity recovery cycles (MVRCs) suggest Na+ /K+ -adenosine triphosphatase activation becomes defective in advanced DM1. We used MVRCs to investigate muscle excitability in DM1 and DM2. MVRCs were measured for patients with mild (n = 8) and advanced (n = 11) DM1, DM2 (n = 4), and normal controls (n = 30). Residual supernormality after multiple conditioning stimuli was increased in DM2 and advanced DM1. Advanced DM1 was distinguished by increases in muscle relative refractory period (MRRP) and reduced early supernormality as well as peak amplitude decrements for the first and last responses in train during repetitive stimulation. Prolongation of the MRRP indicates that depolarization of the resting muscle membrane potential occurs in advanced DM1, with possible implications for future therapeutic approaches. Muscle Nerve 57: 595-602, 2018.
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